Effect of surface modification on silica supported Ti catalysts for cyclohexene oxidation with vapor-phase hydrogen peroxide

Surface modification via grafting of organic moieties on a Lewis acid catalyst (silica supported Ti catalyst, Ti-SiO2) alters the activation of H2O2 in vapor-phase cyclohexene epoxidation. Grafting a fluorous group (1H,1H-perfluoro-octyl) suppresses activity of Ti-SiO2. Conversely, grafting either a nonpolar group (octyl) or a polar aprotic group (triethylene glycol monomethyl ether) enhances rates and shifts selectivity toward trans-1,2-cyclohexanediol.

Post-synthetic modication provides an opportunity to tune the surface properties (e.g.hydrophilicity/hydrophobicity) of previously-synthesized, supported catalysts.For example, surface modication to remove surface hydroxyls and to increase hydrophobicity of a surface is an effective way to reduce the negative effects of water in liquid-phase selective oxidation chemistry. 1,2This increase in hydrophobicity is particularly useful when water adsorbs onto the active metal site competingly with a reactant.Beyond altering surface hydrophilicity/ hydrophobicity, other post-synthetic modications include overcoating of metal oxide layers, 3,4 graing of functional organic groups, 5,6 and depositing additional active metal oxide sites. 7,8or liquid phase reactions, any surface modications must compete against the solvent for any changes to the local environment around the active site.Here, we report graing of organic molecules to change surface properties for vapor phase cyclohexene epoxidation with vaporized H 2 O 2 , where we hypothesized that surface modication might have a more direct impact on the elementary steps of catalysis.By condensing the corresponding terminal alcohols with surface silanols, we graed (Scheme 1) three different types of functional groups on a presynthesized Ti-SiO 2 Lewis acid catalyst: octyl groups (Ti-SiO 2 -o, nonpolar), triethylene glycol monomethyl ether (Ti-SiO 2 -tg, polar aprotic), and 1H,1H-peruoro-1-octyl (Ti-SiO 2 -F, uorous).
We prepared a highly dispersed silica supported Ti catalyst (Ti-SiO 2 ) via liquid-phase graing of trichloro(pentamethylcyclopentadienyl)titanium(IV) onto a mesoporous silica support at 0.2 Ti atoms per nm 2 , followed by calcination, which is known to give high specic activity in H 2 O 2 activation. 9We modied the parent Ti-SiO 2 by graing the corresponding terminal alcohol in reuxing toluene for 24 h, Soxhlet extraction for 24 h in toluene to remove any ungraed species, 2 and drying at 100 °C under vacuum.Successful graing is indicated by slight decreases in BET surface area (Fig. 1(a) and Table 1) without changes to the shape of physisorption isotherm and by mass losses in thermogravimetric analysis (Fig. 1(b)).Mass losses beyond the lowtemperature desorption of water are due to combustion or decomposition of the graed species.The water desorption temperatures of modied catalysts are similar, which agree with the previous study. 2Mass losses beyond the shaded regime in Fig. 1(b) correspond to loadings of 0.75, 0.42, and 0.30 groups per nm 2 , for Ti-SiO 2 -o, Ti-SiO 2 -tg, and Ti-SiO 2 -F, respectively.Graing 1-octanol on a Ta-SiO 2 catalyst was previously reported to give 0.39-0.64groups per nm 2 . 2 Most importantly, all these values are higher than the surface Ti loading of parent supported catalyst, which is 0.2 Ti atoms per nm 2 , so that they should be sufficient to affect catalytic behavior of Ti-SiO 2 .
We performed vapor-phase cyclohexene epoxidation at 120 °C, 3 kPa of cyclohexene, and 3 kPa of vaporized H 2 O 2 employing our custom built reactor. 10Here, we used H 2 O 2 in acetonitrile, dried over MgSO 4 , to minimize initial water content. 11In this study, products were detected with online GC-FID and an in-jet methanizer.We do not observe any C 6 derived products other than cyclohexene epoxide (epoxide) and trans-1,2-cyclohexanediol (diol), consistent with our previous work with Ti-SiO 2 at similar conditions. 10In these systems, cyclohexene rst converts to epoxide, and then hydrolyzes to the trans-diol (Scheme 2).This stepwise conversion of cyclohexene is consistent with our previous studies, 7,10 as we do not observe any cisdiol that is the product of direct cis-dihydroxylation of cyclohexene.Radical oxidation to cyclohexenone or cyclohexenol is  not observed.Background over-oxidation to CO and CO 2 occurs at a rate of approximately 1.2 to 4.1 mol cyclohexene mol Ti −1 h −1 or 0.2 to 0.8% conversion at these conditions, regardless of catalyst.
The parent Ti-SiO 2 shows an initial turnover frequency (TOF = mol (epoxide+diol) mol Ti −1 hr −1 ) of 19.6 h −1 at 50 min time-onstream (TOS), which decays to a steady-state rate of 6.5 h −1 at 600 min TOS.As seen mostly clearly in the selectivity plot, steady-state is reached aer ∼200 minutes, with only slow catalyst deactivation thereaer (Fig. 2).The steady state selectivity is 39%/42% to epoxide and diol respectively, with the remainder going to background overoxidation to CO x .Graing of a uorous group (Ti-SiO 2 -F) almost totally suppresses C 6 product formation.The small amount of remaining C 6 formation has a selectivity of 18%/32% to epoxide and diol, relatively similar to the parent catalyst and suggesting the existence of small patches of unfunctionalized surface.Otherwise, the Scheme 2 Reaction network of cyclohexene epoxidation to cyclohexene oxide (epoxide), and its hydrolysis to yield trans-1,2-cyclohexane diol (diol).Surface modifications near the active site can alter the strength of adsorption of reactants and intermediates, altering product selectivity.conclusion is that the uorous surface makes binding and activation of cyclohexene unfavorable by inhibiting the adsorption of cyclohexene on the surface.Conversely, graing of either nonpolar octyl or polar aprotic tri(ethylene glycol) groups on Ti-SiO 2 increases rates by at least 1.7-fold at steadystate, relative to the parent catalyst.These enhanced catalytic rates have two effects.First, the loss of C 6 to background overoxidation drops dramatically, from 19% in the parent catalyst to 8-11% in the modied catalysts.Moreover, the C 6 selectivity shis to substantially favor hydrolysis of the epoxide to the diol, giving approximately 5%/85% for epoxide and diol, respectively, at steady state.This behavior is quite different from that observed in the condensed phase, where graing groups to remove surface silanols tends to decrease yields slightly and increase epoxide selectivity relative to diol by suppressing water sorption at the active site. 2In the vapor phase and for these wide-pore materials, the lack of a liquid solvent phase means that surface modication can more directly inuence the stability of reaction intermediates.As suggested in Scheme 2, the surface modications appear to be strengthening the adsorption of cyclohexene and the intermediate epoxide, leading to corresponding increases in rate and selectivity to the hydrolysis product.In addition, a recent study by Leonhardt et al.  proposed computationally that an epoxide molecule can remain adsorbed to one facet of the Ti-OH site while still leaving another coordination site available for oxidation of an incoming cyclohexene. 12In that mechanism, enhancing epoxide adsorption at the active site increases hydrolysis to the diol without inhibitingor even enhancingoverall product formation rates, such as we have observed with the octyl-and tg-modied surfaces.Overall, these observations show that surface graing can play a signicant role in modifying the reactivity of catalysts in the nascent eld of selective oxidation with vaporized H 2 O 2 .Also, the results presented here contribute/expand to the current design strategy of post-modication of heterogeneous catalysts with simple method.Additional studies will be carried out to understand the precise mechanistic origins of these changes in rate and selectivity and develop further strategies to tune catalyst surface properties.

Table 1
Summary of catalyst properties and activities a TOF values of production of epoxide and diol values at 600 min.Steady state operation is reached aer 200-400 minutes at these conditions.